1. Field of the Invention
This invention relates to a method for polymerizing halogenated phenolic compounds. More particularly, this invention relates to a method for removing brominated phenolic compounds from a waste stream. The brominated phenolic compounds are polymerized to obtain a fine, innocuous solid which can be easily separated from the waste stream.
2. Description of the Prior Art
Tetrabromobisphenol-A ("TBBPA") also known as 4,4'-isopropylidenebis[2,6-dibromophenol] is a bromine-containing flame-retardant monomer for plastics. TBBA is used in epoxy resins for the copper-clad electronic circuit board market. TBBPA is also used in polycarbonate, polybutylene terephthalate and unsaturated polyester resins.
The preparation of tetrabromobisphenol-A (TBBPA) by brominating bisphenol-A ("BPA") in a solvent is well-known. The brominating agent typically is bromine or a bromine-chlorine mixture. The solvent can be an alcohol, aqueous acetic acid, a non-polar solvent or a two-phase water-organic system.
One method of producing TBBPA is described in U.S. Pat. No. 3,182,088. It involves the bromination of BPA in an anhydrous methanol solvent. Following bromination, TBBPA is crystallized by adding water. The crystals are filtered, washed on the filter to remove impurities and then dried. Generally, an attempt is made to recover the sodium bromide in the waste stream for its bromine value.
If a methanol solvent is used, as exemplified in U.S. Pat. No. 3,182,088, the excess solvent may be recovered upon completion of the reaction using a methanol distillation column. Depending on the process conditions selected, a variety of compounds are present in the aqueous column bottoms. If the methanol distillation is carried out under caustic conditions using an alkali metal hydroxide such as NaOH at a pH of 13-14, the components present in the distillation column bottoms include sodium salts of brominated phenolics, residual methanol, sodium bromide, trace amounts of acetone, NaOH, and the like. If an alkali metal hydroxide is not added prior to the distillation, then the waste stream is acidic and may be neutralized with a base. If NaOH is used then sodium salts of brominated phenolic compounds and sodium salts of polybrominated bisphenol-A are present in the waste stream. The organic bromides found in the distillation column waste stream are primarily polyhalogenated phenolics such as tetrabromobisphenol-A, tribromobisphenol-A and tribromophenol. The average composition of the distillation column bottoms is quite variable. A typical composition for a basic distillation is 92.7-95.4 weight percent water, less than 0.2 weight percent methanol, 0.9-1.1 weight percent brominated phenolic compounds,
3.0-5.0 weight percent NaBr and 0.5-1.0 weight percent NaOH.
The process of the present invention is designed to remove polyhalogenated phenolic compounds from waste streams such as those described above. Treatment of these compounds by the practice of the present invention results in the formation of an insoluble solid polymer that is easily removed by filtration. The precipitate removed is an inert, non-hazardous, clay-like polymer that is suitable for disposal in a sanitary land-fill. At the present time polybrominated phenols are suspected carcinogens but the polybrominated bisphenols are not. The polybrominated phenolic compounds present a waste disposal problem because of their potential impact on health and the environment. It is beneficial to alter or modify the polybrominated phenols in an economically feasible manner to obtain non-hazardous material. The non-hazardous material is then easily disposed of in a landfill site.
A prior method of handling waste stream containing brominated phenolic impurities involved precipitation and incineration. After filtering off the TBBPA, most of brominated phenolic impurities were precipitated from the aqueous acidic filtrate by distilling out most of the methanol; usually, in a flash pot ahead of the final methanol recovery distillation column. The precipitated solids were sticky and hard to handle but they could be dissolved in methanol. Since some of the brominated phenolic wastes are suspected to be carcinogens, disposal of the methanol solution was costly. A waste incerator was used followed by an exhaust gas scrubber. The present process can be applied to the removal of above brominated phenolic waste from the methanol solution without the necessity of incineration.
It has now been discovered that a waste stream containing sodium salts of polybrominated phenolic compounds can be treated with an oxidizing agent such as sodium hypochlorite (NaOCl) or sodium hypobromite (NaOBr). These oxidizing agents initiate a polymerization reaction among the polybrominated phenolic and polybrominated bisphenolic compounds in the waste stream. The polymerized product is a fine, innocuous, insoluble solid that does not present health or environmental problems. The solid waste product settles rapidly and therefore may be readily separated from the aqueous waste stream for disposal.